1. Field of the Invention
The present invention relates to a method and an apparatus for collecting bacterial cells, a method for acclimatizing bacterial cells, and wastewater treatment equipment, in particular, to a technique for collecting anaerobic ammonium oxidizing bacteria used for anaerobic ammonium oxidation and a technique for acclimatizing collected anaerobic ammonium oxidizing bacteria.
2. Description of the Related Art
Nitrogen components contained in sewage and industrial effluent cause eutrophication in lakes and lower the level of dissolved oxygen in rivers, and thus must be removed. Examples of main nitrogen components contained in sewage and industrial effluent include ammonium nitrogen, nitrite nitrogen, nitrate nitrogen, and organic nitrogen.
Conventionally, when nitrogen in such wastewater has a low concentration, ion exchange and oxidation by chlorine or ozone are used to remove nitrogen. When the nitrogen has a medium or high concentration, biological treatment is employed and usually operated in the following conditions.
In the biological treatment, nitrification and denitrification treatment are performed by aerobic nitrification and anaerobic denitrification. In the aerobic nitrification, ammonium nitrogen and nitrite nitrogen are oxidized by ammonium oxidizing bacteria (Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus, etc.),and nitrite oxidizing bacteria(Nitrobactor, Nitrospina, Nitrococcus, Nitrospira,etc). On the other hand, in the anaerobic denitrification, denitrification is performed by heterotrophic bacteria (Pseudomonas denitrificans, etc.).
A nitrification vessel for aerobic nitrification is operated under a load of 0.2 to 0.3 kg-N/m3/day, and a denitrification vessel for anaerobic denitrification is operated under a load of 0.2 to 0.4 kg-N/m3/day. To treat nitrogen contained in a total concentration of 30 to 40 mg/L in sewage, the sewage must be retained in a nitrification vessel for 6 to 8 hours and in a denitrification vessel for 5 to 8 hours, and large-scale treatment vessels are required. Further, a nitrification vessel and a denitrification vessel are designed to be operated under the loads as described above for industrial effluent containing only inorganic substances. However, denitrification of such wastewater requires organic substances, and thus methanol is added at a concentration three to four times of that of nitrogen. For this reason, not only an initial cost but also a large running cost are required.
In this situation, recently, a method for removing nitrogen comprising anaerobic ammonium oxidation has been attracted attention (for example, Japanese Patent Application Publication No. 2001-37467 and Japanese Patent Application Publication No. 2003-24990). This anaerobic ammonium oxidation is a method comprising simultaneously denitrifying ammonium as a hydrogen donor and nitrite as a hydrogen receptor by anaerobic ammonium oxidizing bacteria according to the following reaction formula:1.0NH4+1.32NO2+0.066HCO3+0.13H+→1.02N2+0.26NO3+0.066CH2O0.5N0.15+2.03H2O  (Formula 1)
This method has advantages in that ammonium used as a hydrogen donor can considerably reduce the amount of methanol used for denitrification and reduce the amount of a sludge produced, etc. Thus, the method is considered to be an effective method for removing nitrogen in the future.